Looking at the wiring diagrams in the reference section/years for 1966, it would appear that it is mounted on the firewall shortly after the harness comes out of the firewall:
https://www.teambuick.com/reference/...ng_120-147.php
https://www.teambuick.com/reference/...ng_120-131.php
The entire 1966 Chassis manual is available in the reference section CLICK HERE
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